The formation of galaxies and the origin of the high-velocity hydrogen.

The observations of the high-velocity hydrogen at intermediate and high galactic latitudes are reviewed. The phenomena indicate that intergalactic gas is streaming into the Galaxy. No more than a rough estimate can be made of the total fiow. The inflow provisionally adopted in the article is 2 x 1017 H atoms per cm2 of the galactic plane per million years, corresponding with an increase of the Galaxy's mass of 0.9% per 10 years. The cloudy structure of the high.velocity gas is probably caused by the structure of the halo. An inflow of this order is plausible if one considers the probable origin of spiral galaxies. It is shown that these cannot generally have obtained their angular momentum by the action of external gravitational fields or by collisions in the course of their evolution. Therefore they must have been endowed with their angular momentum from the beginning, and cannot have formed by gravitational instability from large-scale fiuctuations directly after the fireball stage of the universe, as commonly supposed. The universe must thus have had a high degree of turbulence on a galactic scale, as was first proposed by von WeizsILcker. Th order that enough momentum can be contained in a "proto-spiral" the scale of the universe at the time of its inception should have been about 1/30 of the present scale. Initially the protogalaxies presumably expanded with the universe, though at a much reduced rate. Their collapse into actual galaxies should have come much later (at t> 1 x 10 years). The age of most frequent galaxy collapses may well correspond with that of the birth of the majority of powerful radio sources, at z > 1 (Section 7.3). Transition regions at the outer boundaries of the cells from which protogalaxies are formed, as well as natural irregularities in the rotation of these cells will make the infall of gas into the galaxies continue long after the principal collapse. The present-day inflow of intergalactic gas expected from this process of galaxy formation is of the order of that inferred from the observations. Compelling independent evidence for the existence of a great density of intergalactic matter is given by the relative motion of the Andromeda nebula and the Galaxy, as has previously been emphasized by Kahn and Woltjer. The intergalactic gas must have a high temperature. Key words: cosmology - galaxies (origin) - high-velocity gas - local group - universe - galactic halo - radio sources